JP7127735B2 - HOT STAMP MOLDED PRODUCT AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Description

The present invention relates to a hot-stamped molded article and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2019-070212 filed in Japan on April 1, 2019, the content of which is incorporated herein.

In today's highly specialized industrial technology field, materials used in each technical field are required to have special and advanced performance. For example, steel sheets for automobiles are required to have high strength in order to improve fuel efficiency by reducing the weight of automobile bodies, in consideration of the global environment. When the high-strength steel sheet is applied to the vehicle body of an automobile, the thickness of the steel sheet can be reduced to reduce the weight of the vehicle body while imparting desired strength to the vehicle body.

However, in press forming, which is a process for forming vehicle body members for automobiles, the thinner the steel plate used, the more likely cracks and wrinkles are generated. Therefore, steel sheets for automobiles are also required to have excellent press formability.

Ensuring press formability and increasing the strength of the steel sheet are contradictory elements, so it is difficult to satisfy these characteristics at the same time. In addition, when a high-strength steel plate is press-formed, the shape of the member is greatly changed due to springback when the member is taken out of the mold, making it difficult to ensure the dimensional accuracy of the member. Thus, it is not easy to manufacture a high-strength vehicle body member by press molding.

Until now, as a method of manufacturing an ultra-high-strength vehicle body member, for example, as disclosed in Patent Document 1, a technique of press-molding a heated steel plate using a low-temperature press die has been proposed. . This technology is called hot stamping or hot pressing, and because it press-forms steel sheets that have been softened by being heated to high temperatures, it is possible to manufacture members with complex shapes with high dimensional accuracy. can. In addition, since the steel sheet is rapidly cooled by contact with the mold, it is possible to greatly increase the strength by quenching at the same time as press forming. For example, Patent Document 1 describes that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel plate having a tensile strength of 500 to 600 MPa.

Among vehicle body members, frame structural parts such as center pillars and side members are often provided with a hard portion and a soft portion within the member in order to control the deformation state of the member when an automobile collides.

As a method of manufacturing a member having a soft portion by hot stamping, Patent Document 2 discloses a method of partially changing the heating temperature of a steel plate by induction heating or infrared heating to soften the portion heated to a low temperature. ing.
Patent Literature 3 discloses a method of softening a steel plate by partially lowering the heating temperature by attaching a heat insulating material to a portion of the steel plate when the steel plate is heated in a furnace.

Patent Documents 4 and 5 disclose a method of partially changing the cooling rate of a steel sheet by changing the contact area between the steel sheet and the mold during forming, thereby softening the portion where the cooling rate is low. there is
Patent Literature 6 discloses a technique of hot stamping using a so-called tailored blank material in which two raw plates are welded and connected.

In hot stamping, a steel sheet is generally heated to the austenite region and then cooled at a cooling rate equal to or higher than the critical cooling rate to form a single martensitic structure and increase strength. On the other hand, in the methods described in Patent Documents 2 to 5, as described above, the heating temperature or cooling rate of the steel sheet is partially lowered, and a structure other than martensite is partially generated to soften the steel sheet. However, since the fraction of the structure other than martensite changes sensitively to the heating temperature and cooling rate, the methods of Patent Documents 2 to 5 have the problem that the strength of the soft portion is not stable.

In the technique described in Patent Document 6, by using a steel plate with low hardenability as one of the base plates, a soft portion can be formed under certain heating and cooling conditions. However, although the metallographic structure and strength characteristics of the soft portion are highly dependent on the chemical composition of the steel sheet, Patent Document 6 does not give any consideration to the chemical composition of the steel sheet having low hardenability.

In response to such problems, Patent Documents 7 and 8 disclose a method of stabilizing the strength of the soft portion in a hot stamped member consisting of a hard portion and a soft portion, or in a hot stamped member that is entirely soft. It is

Specifically, Patent Document 7 discloses a 600 to 1,200 MPa class high-pressure steel for automobiles in which the C content is restricted to a lower level and a certain amount or more of a quenching element is contained to suppress the formation of ferrite, pearlite, and martensite during cooling. A strength member and method of making the same are disclosed.
In addition, Patent Document 8 discloses a hot stamped member having a tensile strength of 500 MPa or more in which the C content is restricted to a low level and Ti is contained to control the amount of martensite generated, and a method for producing the same. .

According to the techniques described in Patent Documents 7 and 8, it is possible to improve the uniformity of strength and elongation within the member. However, according to the studies of the present inventors, in the techniques described in Patent Documents 7 and 8, the metal structure includes hard structures such as bainite and martensite, so the thermal stability is low, and the member is not coated with baking. It was found that the strength may decrease when the treatment is applied. Automobile members are often subjected to paint baking treatment, so the techniques described in Patent Documents 7 and 8 still have room for improvement.

Japanese Patent Application Laid-Open No. 2002-102980 Japanese Patent Application Laid-Open No. 2005-193287 Japanese Patent Application Laid-Open No. 2009-61473 Japanese Patent Application Laid-Open No. 2003-328031 WO2006/38868 Japanese Patent Application Laid-Open No. 2004-58082 Japanese Patent Application Laid-Open No. 2005-248320 WO2008/132303

As mentioned above, it is not easy to manufacture soft parts or parts containing soft parts by hot stamping. In particular, it has been difficult in the prior art to manufacture a hot-stamped member (molded article) having excellent thermal stability and low strength, which contains a soft portion in part or in its entirety by hot stamping.

The present invention solves the above problems, has excellent thermal stability, more specifically, has a small change in strength (tensile strength) before and after paint baking treatment due to paint baking treatment, and has a tensile strength of 440 MPa. As described above, it is an object of the present invention to provide a hot-stamped molded article having a portion of less than 700 MPa and a method for producing the same.

The present invention has been made to solve the above problems, and the gist thereof is the following hot-stamped molded article and method for producing the same.

(1) A hot-stamped article, in which all or part of the hot-stamped article contains, by mass%, C: 0.001% or more and less than 0.090%, Si: less than 0.50%, and Mn : 0.50% or more and less than 1.70%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 2.500%, N: 0.0200% or less, B: 0.0002 to 0.0200%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0-0.300%, Zr: 0-0.300%, Cr: 0-2.00%, Mo: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00 %, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0500%, and the balance: Fe and impurities The metal structure has, in area%, ferrite: more than 50.0%, tempered martensite: 5.0% or more and less than 50.0%, martensite: 0% or more and less than 10.0%, bainite: 0% or more and less than 20.0%, the tensile strength is 440 MPa or more and less than 700 MPa, and ΔTS, which is the amount of decrease in the tensile strength when subjected to heat treatment at 170 ° C. for 20 minutes, is 100 MPa or less.
(2) The hot stamped product according to (1) above has the chemical composition, in mass%, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.300%, V: 0 .001-0.300%, Zr: 0.001-0.300%, Cr: 0.001-2.00%, Mo: 0.001-2.00%, Cu: 0.001-2.00 %, Ni: 0.001 to 2.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, and Bi: It may contain one or more selected from the group consisting of 0.0001 to 0.0500%.
(3) The hot-stamped article described in (1) or (2) above may have a plating layer on its surface.
(4) A method for producing a hot stamped product according to (1) or (2) above, wherein a steel sheet for hot stamping having the chemical composition according to (1) or (2) above is subjected to Ac ₃ points A heating step of heating to a temperature exceeding , and hot stamping is started at a temperature of (Ar ₃ point - 200 ° C.) or more and less than Ar ₃ point on the hot stamping steel plate after the heating step, followed by 90 and a reheating step of heating the molded article after the hot stamping process to a temperature of 100 to 140° C. and holding it at that temperature for 3 to 120 minutes.
(5) A method for producing a hot stamped product according to (1) or (2) above, wherein a steel plate for hot stamping having the chemical composition according to (1) or (2) above is used as a joining steel plate. A joining step of forming a joined steel plate by joining with, a heating step of heating the joined steel plate after the joining step to a temperature exceeding the Ac ₃ point of the hot stamping steel plate, and the joining steel plate after the heating step , a hot stamping step in which hot stamping is started at a temperature of (Ar ₃ point −200 ° C.) or more and less than Ar ₃ point of the hot stamping steel sheet, and then cooled to a temperature of less than 90 ° C., and after the hot stamping step and a reheating step of heating the molded product of to a temperature of 100 to 140° C. and maintaining it at that temperature for 3 to 120 minutes.
(6) A method for producing a hot-stamped product according to (3) above, wherein the steel sheet for hot stamping has the chemical composition according to (1) or (2) above and has a plating layer on its surface. A heating step of heating to a temperature exceeding Ac ₃ point, and the hot stamping steel plate after the heating step (Ar ₃ point −200 ° C.) or more and Ar ₃ point Start hot stamping at a temperature below. followed by a hot stamping step of cooling to a temperature of less than 90 ° C.; , provided.
(7) A method for producing a hot-stamped product according to (3) above, wherein the steel sheet for hot stamping has the chemical composition according to (1) or (2) above and has a plating layer on its surface. is joined to a joining steel plate to form a joining steel plate, a heating step of heating the joining steel plate after the joining step to a temperature exceeding the Ac ₃ point of the hot stamping steel plate, and after the heating step A hot stamping step of starting hot stamping on the bonded steel sheet at a temperature of (Ar ₃ point - 200 ° C.) or higher and less than Ar ₃ point of the hot stamping steel plate, and then cooling to a temperature of less than 90 ° C. and a reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140° C. and maintaining the temperature at that temperature for 3 to 120 minutes.

According to the present invention, it is possible to obtain a hot-stamped molded product having a portion with a tensile strength of 440 MPa or more and less than 700 MPa, which has a small change in strength (excellent in thermal stability) due to paint baking treatment.

1 is a schematic diagram showing the shape of a hot-stamped article produced in Example 1. FIG. 2 is a schematic diagram showing the shape of a hot-stamped article produced in Example 2. FIG.

The present inventors diligently studied a method for suppressing a reduction in the strength of a hot-stamped article having a tensile strength of 440 MPa or more and less than 700 MPa during paint baking. As a result, the following findings were obtained.

(A) If the metallographic structure of the hot-stamped product contains a large amount of hard structure such as martensite or bainite, the tensile strength of the product is greatly reduced by the paint baking treatment. It is considered that this is because the hard structure is tempered and softened.

(B) On the other hand, even if the hot stamped product has a low hard structure fraction and a metal structure mainly composed of soft structures including ferrite, depending on the chemical composition and hot stamping conditions, the tensile strength may be reduced by paint baking. may drop significantly.

(C) In the hot stamping process, by starting hot stamping in a temperature range where ferrite and austenite coexist, and reheating the hot stamped product after the hot stamping process in a predetermined temperature range , the decrease in tensile strength due to paint baking treatment is suppressed.

Although the reason for this is not clear, the inventors presume that it is due to the following reason. (a) In hot-stamped products, solid-solution carbon contained in ferrite precipitates as coarse iron carbides when the paint is baked, causing a decrease in ferrite strength. (b) In hot stamped products, fine iron carbides or fine iron carbon clusters change into coarse iron carbides during paint baking, which causes a decrease in ferrite strength. (c) When hot stamping is performed in the presence of ferrite, dislocations are introduced into the ferrite in the hot stamped product. (d) When the hot-stamped product is reheated, solute carbon in ferrite precipitates on dislocations, the amount of solute carbon decreases, and fine iron carbides or fine iron-carbon clusters coarsen.

(D) When B (boron) is included in the chemical composition, the decrease in tensile strength due to paint baking treatment is suppressed. Although the reason for this is not clear, the inclusion of B increases the amount of dislocations introduced into the ferrite in hot stamped products, promotes the precipitation of carbides due to reheating, and further reduces the amount of dissolved carbon. and that the coarsening of iron carbides or iron-carbon clusters is promoted.

From the knowledge of (A) to (D) above, the present inventors used a steel plate containing a desired amount of B (boron), started hot stamping in a temperature range where ferrite and austenite coexist, and further , By reheating the hot stamped molded product after the hot stamping process, it has a metal structure mainly composed of ferrite, has a tensile strength of at least a part of less than 700 MPa, has excellent thermal stability, and is coated. It was found that a hot-stamped molded product can be produced with little reduction in strength due to baking.

Hereinafter, each requirement of the hot stamped article according to one embodiment of the present invention (the hot stamped article according to the present embodiment) and the manufacturing method thereof will be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.

<Chemical composition of hot stamped product>
All or part of the hot stamped article according to this embodiment has the chemical composition shown below. The reasons for limiting each element are as follows. In the following description, "%" for contents in chemical compositions all means "% by mass". When the hot stamped article has a portion with a tensile strength of less than 700 MPa and a portion with a tensile strength of 700 MPa or more, at least the portion with a tensile strength of less than 700 MPa has the following chemical composition: It's fine if you do.

C: 0.001% or more and less than 0.090% C is an element that has the effect of increasing the tensile strength of the steel sheet after hot stamping (the steel sheet included in the hot stamped product). If the C content is less than 0.001%, an increase in tensile strength by hot stamping cannot be expected. Therefore, the C content is made 0.001% or more. A preferable C content is 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more.
On the other hand, when the C content is 0.090% or more, the area ratio of tempered martensite, martensite, and/or bainite increases in the metal structure after hot stamping, and the tensile strength of the hot stamped product increases. It becomes 700 MPa or more, and the thermal stability of the hot stamped product cannot be ensured. Therefore, the C content should be less than 0.090%. A preferred C content is less than 0.085%, less than 0.080%, less than 0.075%, or less than 0.070%.

Si: less than 0.50% Si is an element contained as an impurity in steel. If the Si content is 0.50% or more, it becomes difficult to ensure the thermal stability of the hot stamped product even if the hot stamped product is subjected to reheating treatment as described later. Therefore, the Si content should be less than 0.50%. A preferred Si content is less than 0.40%, less than 0.20%, less than 0.10%, or less than 0.05%. When a plated steel sheet is used as the steel sheet for hot stamping, the Si content is preferably less than 0.40%, more preferably less than 0.30%, in order to ensure plateability.
Although the lower limit of the Si content is not particularly limited, an excessive decrease in the Si content causes an increase in steelmaking costs. Therefore, it is preferable to set the Si content to 0.001% or more. Moreover, since Si has the effect of increasing the tensile strength of the steel sheet after hot stamping, it may be positively contained. From the viewpoint of increasing the strength, the Si content is preferably 0.10% or more, or 0.20% or more.

Mn: 0.50% or more and less than 1.70% Mn is an element that improves the hardenability of steel, and is contained in an amount of 0.50% or more in order to obtain a metal structure containing ferrite and tempered martensite. A preferable Mn content is 0.60% or more, or 0.70% or more.
On the other hand, when the Mn content is 1.70% or more, it becomes difficult to ensure the thermal stability of the hot stamped product even if the hot stamped product is subjected to reheating treatment as described later. Therefore, the Mn content should be less than 1.70%. The Mn content is preferably less than 1.50%, less than 1.20%, less than 1.00%, or less than 0.80%.

P: 0.200% or less P is an element contained as an impurity in steel. If the P content exceeds 0.200%, the weldability and toughness after hot stamping deteriorate significantly, so the P content is made 0.200% or less. A preferable P content is 0.100% or less, 0.050% or less, or 0.020% or less.
Although the lower limit of the P content is not particularly limited, an excessive decrease in the P content causes an increase in steelmaking costs. Therefore, it is preferable to set the P content to 0.001% or more. Moreover, since P has the effect of increasing the tensile strength of the molded product after hot stamping, it may be positively contained. From the viewpoint of increasing the strength, the preferable P content is 0.010% or more, 0.020% or more, or 0.030% or more. When a plated steel sheet is used as a steel sheet for hot stamping, the P content is preferably 0.050% or less, more preferably 0.040% or less, in order to ensure plating properties.

S: 0.0200% or less S is an element contained as an impurity in steel and embrittles the steel. Therefore, the lower the S content, the better. However, if the S content exceeds 0.0200%, embrittlement of the steel becomes remarkable, so the S content is made 0.0200% or less. A preferred S content is 0.0100% or less, 0.0050% or less, or 0.0030% or less.
Although the lower limit of the S content is not particularly limited, an excessive reduction in the S content causes an increase in steelmaking costs. Therefore, it is preferable to set the S content to 0.0001% or more.

sol. Al: 0.001-2.500%
Al is an element that acts to deoxidize molten steel. sol. If the Al content (acid-soluble Al content) is less than 0.001%, deoxidation will be insufficient. Therefore, sol. Al content is 0.001% or more. sol. The Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more.
On the other hand, sol. If the Al content is too high, the transformation point rises and it becomes difficult to heat the steel sheet to a temperature exceeding the Ac ₃ point in the heating process of hot stamping. Therefore, sol. Al content is 2.500% or less. sol. The Al content is preferably less than 0.500%, less than 0.100%, less than 0.050% or less than 0.040%.

N: 0.0200% or less N is an element that is contained as an impurity in steel and forms nitrides during continuous casting of steel. Since this nitride deteriorates the toughness after hot stamping, the lower the N content is, the better. When the N content exceeds 0.0200%, toughness deteriorates significantly. Therefore, the N content is made 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080% or less than 0.0050%.
Although the lower limit of the N content is not particularly limited, excessive reduction in the N content causes an increase in steelmaking costs, so the N content is preferably 0.0010% or more.

B: 0.0002 to 0.0200%
B is an element that acts to improve the thermal stability of a hot-stamped product having a metallographic structure containing ferrite and tempered martensite. If the B content is less than 0.0002%, the above effects cannot be sufficiently obtained. Therefore, the B content should be 0.0002% or more. A preferable B content is 0.0006% or more, 0.0010% or more, or 0.0015% or more.
On the other hand, when the B content exceeds 0.0200%, the tensile strength of the steel sheet after hot stamping becomes too high, and the thermal stability of the hot stamped product deteriorates. Therefore, the B content should be 0.0200% or less. A preferred B content is less than 0.0050%, less than 0.0030%, or less than 0.0020%.

Ti: 0-0.300%
Nb: 0-0.300%
V: 0-0.300%
Zr: 0-0.300%
Ti, Nb, V and Zr are elements that have the effect of increasing the tensile strength of the hot stamped product by refining the metal structure. In order to obtain this effect, one or more selected from Ti, Nb, V and Zr may be contained as necessary. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

To obtain the above effects, it is preferable to contain at least one selected from Ti, Nb, V and Zr in an amount of 0.001% or more. Further, it is more preferable to contain at least one of Ti of 0.005% or more, Nb of 0.005% or more, V of 0.010% or more, and Zr of 0.005% or more.

When Ti is contained, the Ti content is more preferably 0.020% or more, particularly preferably 0.030% or more.
When Nb is contained, the Nb content is more preferably 0.020% or more, particularly preferably 0.030% or more.
When V is contained, the V content is more preferably 0.020% or more.
When Zr is contained, the Zr content is more preferably 0.010% or more.

On the other hand, when the contents of Ti, Nb, V and Zr each exceed 0.300%, the effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when these elements are contained, the contents of Ti, Nb, V and Zr are each set to 0.300% or less.

Also, when the contents of Ti, Nb, V and Zr are high, a large amount of carbides of these elements may be precipitated and the toughness after hot stamping may be impaired.
Therefore, the Ti content is preferably less than 0.060%, more preferably less than 0.040%.
The Nb content is preferably less than 0.060%, more preferably less than 0.040%.
The V content is preferably less than 0.200%, more preferably less than 0.100%.
The Zr content is preferably less than 0.200%, more preferably less than 0.100%.

Cr: 0-2.00%
Mo: 0-2.00%
Cu: 0-2.00%
Ni: 0-2.00%
Cr, Mo, Cu and Ni have the effect of increasing the tensile strength of hot stamped products (steel sheets after hot stamping). Therefore, one or more selected from Cr, Mo, Cu and Ni may be contained as necessary. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

To obtain the above effect, it is preferable to contain at least one selected from Cr, Mo, Cu and Ni in an amount of 0.001% or more. The preferred Cr content is 0.05% or more, the preferred Mo content is 0.05% or more, the preferred Cu content is 0.10% or more, and the preferred Ni content is 0.10% or more. be.

On the other hand, when the contents of Cr, Mo, Cu and Ni each exceed 2.00%, the tensile strength of the steel sheet after hot stamping becomes too high and the thermal stability of the hot stamped product deteriorates. do.
Therefore, even when the above elements are contained, the contents of Cr, Mo, Cu and Ni are each set to 2.00% or less. Preferred Cr content is less than 0.50% or less than 0.20%, preferred Mo content is less than 0.50% or less than 0.20%, preferred Cu content is less than 1.00% and the preferred Ni content is less than 1.00%.

Ca: 0-0.0100%
Mg: 0-0.0100%
REM: 0-0.1000%
Ca, Mg and REM are elements that have the effect of improving the toughness after hot stamping by adjusting the shape of inclusions. Therefore, one or more selected from Ca, Mg and REM may be contained as necessary. Since these elements do not have to be contained, the lower limit of the content of these elements is 0%.

In order to obtain the above effect, it is preferable to contain at least one selected from Ca, Mg and REM each in an amount of 0.0001% or more.
On the other hand, when the content of Ca or Mg exceeds 0.0100%, or when the content of REM exceeds 0.1000%, the above effects are saturated and the manufacturing cost of the steel plate increases. Therefore, even when the above elements are contained, the contents of Ca and Mg are each set to 0.0100% or less, and the REM content is set to 0.1000% or less.

In this embodiment, REM refers to a total of 17 elements of Sc, Y and lanthanides, and REM content means the total content of these elements. Lanthanides are added industrially in the form of misch metals.

Bi: 0 to 0.0500%
Bi is an element that has the effect of improving the toughness after hot stamping by refining the solidified structure. Therefore, Bi may be contained as necessary. Since Bi does not have to be contained, the lower limit of the Bi content is 0%.

To obtain the above effects, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, still more preferably 0.0005% or more.
On the other hand, when the Bi content exceeds 0.0500%, the above effects are saturated, and the manufacturing cost of the steel sheet increases. Therefore, even when Bi is contained, the Bi content should be 0.0500% or less. The Bi content is preferably 0.0100% or less, more preferably 0.0050% or less.

In the above chemical composition, the balance is Fe and impurities. Here, the term "impurities" refers to ingredients such as ores, scraps, and other raw materials and various factors in the manufacturing process that are mixed in when steel sheets are industrially manufactured. It means what is allowed as long as it does not give

The chemical composition of the hot-stamped article described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol. Al can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

<Metal structure of hot stamped product>
The metal structure of the hot-stamped product according to this embodiment will be described. All or part of the hot stamped product according to the present embodiment has a metallographic structure containing ferrite, tempered martensite, martensite, and bainite in the following amounts. In the following description of metal structure, "%" means "area %".

Ferrite: More than 50.0% When the area ratio of ferrite is 50.0% or less, the tensile strength of the molded product after hot stamping becomes 700 MPa or more, and thermal stability cannot be ensured. Therefore, the area ratio of ferrite is set to more than 50.0%. The ferrite area ratio is preferably above 60.0%, more preferably above 70.0%, and even more preferably above 80.0%.
Although the upper limit of the area ratio of ferrite does not need to be specified in particular, it is preferably less than 95.0%, more preferably less than 90.0%, in order to increase the strength of the hot stamped product. Less than 0.0% is more preferable.

In the present embodiment, ferrite includes, in addition to polygonal ferrite, quasi-polygonal ferrite and granular bainitic ferrite having a higher dislocation density than polygonal ferrite, and acicular ferrite having serrated grain boundaries. From the viewpoint of thermal stability, the ratio of polygonal ferrite to the entire ferrite is preferably 5.0% or more in terms of area ratio.

Tempered martensite: 5.0% or more and less than 50.0% Tempered martensite is a structure that has the effect of increasing the strength of the hot stamped product while maintaining the thermal stability of the hot stamped product. If the area ratio of tempered martensite is less than 5.0%, the above effect cannot be obtained sufficiently, and the thermal stability and/or the strength of the hot stamped product cannot be ensured. becomes difficult. Therefore, the area ratio of tempered martensite is set to 5.0% or more. The area ratio of tempered martensite is preferably 8.0% or more, more preferably 10.0% or more, and still more preferably 12.0% or more.

On the other hand, if the area ratio of tempered martensite is 50.0% or more, the tensile strength of the steel sheet after hot stamping becomes too high, and the thermal stability of the hot stamped product deteriorates. Therefore, the area ratio of tempered martensite is set to less than 50.0%. The area fraction of tempered martensite is preferably less than 40.0%, more preferably less than 30.0%, even more preferably less than 20.0%.

Martensite: 0% or more and less than 10.0% Bainite: 0% or more and less than 20.0% Metal structure (microstructure) is martensite (untempered martensite, also called fresh martensite) And if bainite is included in a large amount, the thermal stability of the hot-stamped product is deteriorated. Therefore, the area ratio of martensite is set to less than 10.0%, and the area ratio of bainite is set to less than 20.0%. The area ratio of martensite is preferably less than 5.0%, more preferably less than 2.0%, and even more preferably less than 1.0%. The area ratio of bainite is preferably less than 10.0%, more preferably less than 5.0%, and even more preferably less than 2.0%.

Since martensite and bainite do not necessarily need to be contained, the lower limits of the area ratios of martensite and bainite are both 0%.
However, since martensite and bainite have the effect of increasing the strength of hot stamped products, they may be contained in the metallographic structure within the above range. If the area ratios of martensite and bainite are both less than 0.1%, the above effects cannot be sufficiently obtained. Therefore, when increasing the strength, the lower limits of the area ratios of martensite and bainite are preferably 0.1% or more, more preferably 0.5% or more.

The remainder of the metallographic structure may contain pearlite or retained austenite, and may also contain precipitates such as cementite. Since it is not necessary to positively contain pearlite, retained austenite and precipitates, the lower limits of the area ratios of pearlite, retained austenite and precipitates are all 0%.

Since perlite has the effect of increasing the strength of hot stamped products, when increasing the strength, the perlite area ratio is preferably 1.0% or more, more preferably 2.0% or more. Preferably, it is more preferably 5.0% or more.
On the other hand, if the pearlite content is excessive, the toughness after hot stamping deteriorates. Therefore, the area ratio of pearlite is preferably 20.0% or less, more preferably 10.0% or less.

Retained austenite has the effect of improving the impact absorption of hot stamped products. Therefore, when obtaining this effect, the area ratio of retained austenite is preferably 0.5% or more, more preferably 1.0% or more.
On the other hand, an excessive amount of retained austenite lowers the toughness after hot stamping. Therefore, the area ratio of retained austenite is preferably less than 3.0%, more preferably less than 2.0%.

In this embodiment, the area ratio of each metal structure is obtained as follows.
First, a test piece is taken from a hot-stamped product, and after polishing the thickness cross section (longitudinal cross section of the steel sheet), in the case of a non-coated steel sheet, a depth position of 1/4 of the thickness of the steel sheet from the surface of the steel sheet (steel sheet 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface of the steel plate), in the case of plated steel plate, from the boundary between the base steel plate and the plating layer to the plate of the base steel plate Structure observation at a depth position of 1/4 of the thickness (1/8 depth of the plate thickness of the steel plate as the base material from the above boundary to 3/8 depth of the plate thickness of the steel plate as the base material from the above boundary) do. If the hot stamped product has a portion with a tensile strength of less than 700 MPa and a portion with a tensile strength of 700 MPa or more, a test piece is taken from the portion with a tensile strength of less than 700 MPa and observed. I do.

Specifically, after nital corrosion or electrolytic polishing of the polished plate thickness cross section, the structure is observed using an optical microscope and a scanning electron microscope (SEM), and the obtained structure photograph is subjected to image analysis. , the area fractions of ferrite, pearlite, bainite, and tempered martensite are obtained. After that, the same observation position was subjected to repeller corrosion, and then the structure was observed using an optical microscope and a scanning electron microscope (SEM). Calculate the total area fraction of martensite.

In addition, for the same observation position, after electropolishing the plate thickness cross-section, the area ratio of retained austenite is measured using an SEM equipped with an electron beam backscattering pattern analyzer (EBSP).

Based on these results, the area fractions of ferrite, pearlite, bainite, tempered martensite, martensite, and retained austenite are obtained.
It should be noted that tempered martensite can be distinguished from martensite in that iron carbides are present inside, and can be distinguished from bainite in that the iron carbides present inside are elongated in multiple directions. can be done.

<Strength of hot stamped product>
All or part of the hot stamped product according to this embodiment has a tensile strength of 440 MPa or more and less than 700 MPa. For this purpose, the tensile strength of all or part of the base steel plate of the hot stamped product according to the present embodiment must be 440 MPa or more and less than 700 MPa. If the tensile strength is 700 MPa or more, the thermal stability of the hot stamped product cannot be ensured. Therefore, the tensile strength of all or part of the hot stamped product is set to less than 700 MPa. Preferably, all or part of the hot stamped article has a tensile strength of less than 650 MPa, or less than 600 MPa. On the other hand, in order to improve the impact absorption of the hot stamped product, the tensile strength of all or part of the hot stamped product is set to 440 MPa or more. Preferably, all or part of the hot stamped article has a tensile strength of 460 MPa or more, 490 MPa or more, or 540 MPa or more.

In the hot stamped product according to the present embodiment, a soft portion having a tensile strength of 440 MPa or more and less than 700 MPa and a hard portion having a tensile strength of 700 MPa or more may be mixed in the hot stamped product. . By providing regions with different strengths, it is possible to control the deformation state of the hot stamped product at the time of collision, and improve the impact absorption of the hot stamped product. A hot-stamped product having portions with different strengths can be produced by joining two or more types of steel plates having different chemical compositions and then hot-stamping them, as will be described later.

<Thermal stability of hot stamped products>
The hot-stamped product according to the present embodiment has a decrease in tensile strength (ΔTS) of 100 MPa or less compared to the tensile strength before heat treatment when heat-treated at 170° C. for 20 minutes. ΔTS is preferably 60 MPa or less, more preferably 30 MPa or less. Although the lower limit of ΔTS is not particularly limited, it is preferably 1 MPa or more, 5 MPa or more, or 10 MPa or more from the viewpoint of steel sheet manufacturability.

The reason why the strength of a hot stamped product having a structure mainly composed of ferrite (over 50.0% in terms of area ratio) decreases when the paint is baked is that the solid solution carbon present in the ferrite is removed by the paint baking process. Precipitation as coarse iron carbides and fine iron carbides or fine iron carbon clusters present in the ferrite are considered to be caused by the change into coarse iron carbides by the heat treatment during paint baking. Although it is not easy to directly and quantitatively evaluate the existence state of this solid solution carbon and fine iron carbides or fine iron carbon clusters, the tensile strength when heat treated at 170 ° C. for 20 minutes can be indirectly evaluated by the amount of decrease in (ΔTS). If ΔTS is 100 MPa or less, the amount of dissolved carbon in ferrite and the amount of fine iron carbides or fine iron-carbon clusters produced are low, and it is judged that the thermal stability is excellent.

Tensile strength is obtained by extracting a JIS13B tensile test piece and performing a tensile test at a tensile speed of 10 mm/min.

<Plating layer>
The hot stamped product according to this embodiment may have a plating layer on its surface. By providing a plating layer on the surface, it is possible to prevent the formation of scale during hot stamping and further improve the corrosion resistance of the hot stamped product. The type of plating is not particularly limited as long as it meets the above purpose. A hot-stamped article having a plated layer can be obtained by hot-stamping a plated steel sheet, as described later. Hot-stamped products having a coating layer include zinc-based plated steel sheets or aluminum-based plated steel sheets, specifically, for example, hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, hot-dip aluminum-plated steel sheets, and hot-dip Zn-Al alloy plating. Hot-stamped zinc-based coating layer or A hot stamp molded article having an aluminum-based plating layer is exemplified. The plating layer may be formed on one side or both sides.

Next, a hot stamping steel sheet suitable for producing the above hot stamped product will be described.

<Chemical Composition of Steel Plate for Hot Stamping>
Since hot stamping does not substantially change the chemical composition, the chemical composition of the hot stamping steel sheet has the same chemical composition as the hot stamped product described above.

<Metal structure of steel plate for hot stamping>
The metallographic structure of the steel sheet for hot stamping according to the present embodiment preferably contains iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) satisfies the following formula (i).

[Mn] _θ + [Cr] _θ > 1.7 (i)
However, the meaning of each symbol in the above formula is as follows.
[Mn] _θ : Mn content (atomic %) in the iron carbide when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic %
[Cr] _θ : Cr content (atomic %) in the iron carbide when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic %

When the chemical composition of the iron carbide contained in the metal structure of the hot stamping steel sheet satisfies the above formula (i), the thermal stability of the steel sheet after hot stamping can be further improved. The left-side value of the above formula (i) is preferably greater than 3.0, more preferably greater than 4.0.

On the other hand, in order to increase the Mn content and the Cr content in the iron carbide, it is necessary to anneal the hot-rolled steel sheet at a high temperature in the hot-rolled steel sheet annealing process described later, which impairs the manufacturability of the steel sheet. Therefore, the left-side value of the above formula (i) is preferably less than 20.0, more preferably less than 10.0.

In this embodiment, the chemical composition of iron carbide is measured by the following procedure.
First, a test piece is taken from an arbitrary position on the steel plate, and after polishing the plate thickness cross section (longitudinal cross section) parallel to the rolling direction of the steel plate, a position 1/4 depth of the plate thickness from the steel plate surface (plate Precipitates are extracted by a replica method in a region from a depth of 1/8 of the thickness to a depth of 3/8 of the thickness from the surface of the steel plate. This precipitate is observed using a transmission electron microscope (TEM), and the precipitate is identified and compositionally analyzed by electron diffraction and energy dispersive X-ray analysis (EDS).

Quantitative analysis of iron carbides by EDS is performed for three elements, Fe, Mn and Cr, and the Mn content (atomic %) and Cr content (atomic %) when their total content is 100 atomic % , as [Mn] _θ and [Cr] _θ , respectively. This quantitative analysis is performed for a plurality of iron carbides, and the average value is taken as the Mn content and Cr content in the iron carbides in the steel sheet. The number of iron carbides to be measured is 10 or more, and the larger the number of measurements, the better. Iron carbide includes not only cementite that constitutes pearlite, but also cementite that exists in isolation in the metal structure.

In the present embodiment, in the case of a hot-rolled annealed steel sheet, a cold-rolled steel sheet, or an annealed steel sheet, the depth position of 1/4 of the plate thickness from the steel plate surface (1/8 depth of the plate thickness from the steel plate surface to the plate thickness from the steel plate surface) 3/8 depth area), in the case of a plated steel sheet, the 1/4 depth position of the thickness of the base steel sheet from the boundary between the base steel sheet and the coating layer (from the above boundary to the base steel sheet 1/8 depth of plate thickness to 3/8 depth of plate thickness of the base steel plate from the above boundary), the above-mentioned metallographic structure is defined.

The area ratio of the iron carbide does not need to be specified in particular, but in order to increase the tensile strength by refining the metal structure after hot stamping, the area ratio of the iron carbide is preferably 1% or more, and 3% or more. is more preferable.
On the other hand, if the area ratio of iron carbide is excessive, the tensile strength of the steel sheet after hot stamping becomes too high and the thermal stability is impaired. Therefore, the area ratio of iron carbide is preferably 20% or less, more preferably 15% or less.

The metal structure of the steel sheet for hot stamping according to the present embodiment is preferably mainly composed of ferrite, but may contain martensite, tempered martensite, bainite, and retained austenite as the balance, and may further contain iron carbide other than iron carbide. may contain precipitates of However, since martensite, tempered martensite, bainite and retained austenite deteriorate the toughness after hot stamping, the smaller the area ratio of these structures, the better. The area ratios of martensite, tempered martensite, bainite and retained austenite are each preferably less than 1.0%, more preferably less than 0.5%.

The area ratio in the metal structure of the steel sheet for hot stamping can be obtained by the same method as in the case of the hot stamped product.
Although the tensile strength of the steel sheet for hot stamping is not particularly limited, it is preferably 300 MPa or more or 340 MPa or more from the viewpoint of steel sheet manufacturability, and 650 MPa or less or less than 590 MPa from the viewpoint of steel sheet cuttability. preferable.

<Manufacturing method>
A preferred method for manufacturing the hot stamped product according to this embodiment and the steel sheet for hot stamping according to this embodiment will be described.

[Manufacturing method of hot stamped product]
The method for manufacturing a hot stamped product according to the present embodiment includes a heating step of heating the steel sheet for hot stamping having the chemical composition described above, hot stamping the heated steel sheet for hot stamping, and then cooling. A hot stamping process and a reheating process of reheating the molded product after the hot stamping process are included. In the hot stamping process, molding and cooling are performed using a mold to obtain a hot stamped product.

In the heating step of heating the steel sheet for hot stamping, the heating temperature is set to Ac ₃ points or higher. The Ac ₃ point is the temperature at which ferrite disappears in the metal structure when the material steel sheet is heated, and can be obtained from the change in thermal expansion of the steel sheet during the heating process. If the heating temperature is less than the Ac ₃ point, the dissolution of the carbide during heating becomes insufficient and the strength of the hot stamped product decreases. The heating temperature is preferably (Ac ₃ point + 20°C) or higher, more preferably (Ac ₃ point + 40°C) or higher.
Moreover, the steel sheet for hot stamping to be heated preferably has the above structure.

The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, the austenite will coarsen and the strength of the hot stamped product will decrease. Therefore, the heating temperature is preferably 1000° C. or lower, more preferably 950° C. or lower, and even more preferably 900° C. or lower.
Moreover, the holding time at the heating temperature of the hot stamp is preferably 1 to 5 minutes.

In the hot stamping step of performing hot stamping on a heated steel sheet for hot stamping, the starting temperature of hot stamping should be (Ar ₃ point - 200°C) or higher and less than Ar ₃ point. The Ar ₃ point is the temperature at which ferrite begins to form in the metal structure when the material steel sheet is cooled from a temperature exceeding the Ac ₃ point. The Ar ₃ point is obtained from the change in thermal expansion when the steel sheet is cooled after the heating process. If the hot stamping start temperature is above the Ar ₃ point, the amount of dislocations introduced into the ferrite will be insufficient and the thermal stability of the hot stamped product will be impaired. If the hot stamping start temperature is less than (Ar ₃ point -200°C), the area ratio of tempered martensite in the metallographic structure of the hot stamped product decreases, resulting in insufficient strength of the hot stamped product. Preferred upper limits for the hot stamping initiation temperature are less than (Ar ₃ point-20° C.), less than (Ar ₃ point-40° C.), or less than (Ar ₃ point-60° C.). A preferable lower limit of the hot stamping start temperature is (Ar ₃ point-170° C.) or higher, (Ar ₃ point-140° C.) or higher, or (Ar ₃ point-110° C.) or higher.

After molding by hot stamping, the molded article is brought to a temperature of less than 90°C by holding the molded article in the mold and/or removing the molded article from the mold and cooling in any manner. Cooling. If the cooling stop temperature is 90° C. or higher, the area ratio of tempered martensite in the metallographic structure of the hot-stamped product decreases, and the strength of the hot-stamped product becomes insufficient. The cooling stop temperature is preferably less than 50°C, more preferably room temperature. In order to increase productivity, it is preferable to keep the temperature within the mold up to a temperature of less than 90°C.

In the reheating step of reheating the hot stamped product, the reheating temperature is set to 100 to 140° C., and the holding time at the reheating temperature is set to 3 to 120 minutes. If the reheating temperature is less than 100° C., fine iron carbides or fine iron-carbon clusters are produced, which deteriorates the thermal stability of the hot-stamped product. On the other hand, if the reheating temperature exceeds 140°C, the strength of the hot-stamped product will be reduced.

If the holding time is less than 3 minutes, a large amount of dissolved carbon remains in the ferrite in the metallographic structure of the hot-stamped product, degrading the thermal stability of the hot-stamped product. On the other hand, if the holding time exceeds 120 minutes, the strength of the hot-stamped article will decrease. The holding time is preferably adjusted according to the reheating temperature. When the reheating temperature is 100° C. or higher and lower than 120° C., the holding time is preferably longer than 60 minutes, longer than 70 minutes, or longer than 80 minutes. Also, the retention time is preferably less than 110 minutes, less than 100 minutes, or less than 90 minutes. When the reheat temperature is 120-140 minutes, the holding time is preferably greater than 5 minutes, 7 minutes, or 9 minutes, and the holding time is preferably less than 30 minutes, less than 20 minutes, or less than 15 minutes. be. From the viewpoint of productivity, it is preferable to set the reheating temperature to 120 to 140°C.

Further, another method for manufacturing a hot stamped product according to the present embodiment includes a joining step of joining a steel plate (hot stamping steel plate) having the above-described chemical composition to a joining steel plate to form a joined steel plate; A step of heating a bonded steel plate, followed by a step of hot stamping the heated bonded steel plate, followed by cooling, and a step of reheating the hot stamped product after the hot stamping step. include. As a joining method, for example, a method in which a steel plate for hot stamping and a steel plate for joining are butted or overlapped and joined by welding.

The above-mentioned bonded steel sheets are heated to a temperature exceeding the Ac ₃ point of the hot stamping steel sheet, hot stamping is started at a temperature of the hot stamping steel sheet (Ar ₃ point −200 ° C.) or higher and less than the Ar ₃ point, and then Cool to a temperature below 90°C. The hot stamped article is then reheated to a temperature of 100-140° C. and held at that temperature for 3-120 minutes. Preferred heating temperature in the step of heating the bonded steel plate, preferred hot stamping start temperature and preferred cooling stop temperature in the step of hot stamping the joined steel plate, and preferred reheating temperature and preferred holding time in the step of reheating the hot stamped product is the same as the manufacturing method of the hot stamped product that does not include the bonding step, and the reasons are also the same as in the case that the bonded steel plate is not included.

The chemical composition and mechanical properties of the joining steel plate are not particularly limited. However, in order to increase the impact absorption energy of the hot stamped product, it is preferable that the steel plate for joining has a tensile strength of 700 MPa or more after reheating. More preferably, the tensile strength of the joining steel plate after reheating is over 1000 MPa, over 1200 MPa, or over 1500 MPa.

In order to secure the tensile strength of the joining steel plate after hot stamping, the C content of the joining steel plate is preferably 0.090% or more. More preferably, it is 0.100% or more, 0.120% or more, or 0.200% or more. For the same reason, the Mn content of the joining steel plate is preferably 0.50% or more. More preferably, it is 0.80% or more, 1.00% or more, or 1.20% or more.

It is preferable that the steel plate (hot stamping steel plate) used as the above raw material is subjected to hot-rolled plate annealing as described later. Cold rolling, or cold rolling and continuous annealing may be performed after hot-rolled sheet annealing. On the other hand, as steel sheets for joining, there are hot-rolled steel sheets, cold-rolled steel sheets obtained by subjecting hot-rolled steel sheets to cold rolling, hot-rolled steel sheets obtained by subjecting hot-rolled steel sheets to annealing, and cold-rolled steel sheets obtained by subjecting cold-rolled steel sheets to annealing. Any of the annealed steel sheets may be used.

In order to improve the corrosion resistance of the hot-stamped product, a plated steel sheet having a plated surface may be used as the hot-stamping steel sheet and the joining steel sheet. The type of plated steel sheet is not particularly limited, but hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-plated steel sheet, hot-dip Zn-Al alloy plated steel sheet, hot-dip Zn-Al-Mg alloy-plated steel sheet, hot-dip Zn-Al- Examples include Mg--Si alloy plated steel sheet, electro-galvanized steel sheet, and electro-Ni--Zn alloy plated steel sheet.

[Manufacturing method of steel plate for hot stamping]
The steel sheet for hot stamping according to the present embodiment is produced by subjecting the slab having the chemical composition described above to hot rolling and then winding it in a temperature range of 800° C. or less to obtain a hot rolled steel sheet. and a hot-rolled sheet annealing step of heating the hot-rolled steel sheet to a temperature range exceeding 650° C. to obtain a hot-rolled annealed steel sheet.

In the hot rolling step, the coiling temperature after hot rolling is preferably 800° C. or lower. If the coiling temperature exceeds 800°C, the metal structure of the hot-rolled steel sheet becomes excessively coarse, and the tensile strength of the hot-stamped steel sheet decreases. More preferred coiling temperatures are less than 650°C, less than 600°C, or less than 550°C. Also, if the coiling temperature is too low, the hot-rolled steel sheet becomes hard and cold rolling becomes difficult, so the coiling temperature is preferably 400° C. or higher.

The method of manufacturing the slab used in the method of manufacturing the steel sheet for hot stamping according to the present embodiment is not particularly limited. In the preferred slab manufacturing method exemplified, the steel having the above-described composition (chemical composition) is melted by known means and then made into a steel ingot by continuous casting, or by any casting method. It is made into a steel billet by a method such as blooming after making a steel ingot. In the continuous casting process, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to cause the molten steel to undergo external additional flow such as electromagnetic stirring within the mold. The steel ingot or billet that has been cooled once may be reheated and subjected to hot rolling. Alternatively, the steel may be subjected to hot rolling after being kept warm or subjected to auxiliary heating. In the present embodiment, such steel ingots and billets are collectively referred to as "slabs" as materials for hot rolling.

The temperature of the slab subjected to hot rolling is preferably less than 1250°C, more preferably less than 1200°C, in order to prevent coarsening of austenite. Hot rolling is preferably completed in a temperature range of Ar ₃ or more in order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after the completion of rolling.

When hot rolling consists of rough rolling and finish rolling, the rough rolled material may be heated between the rough rolling and finish rolling in order to complete the finish rolling at the above temperature. At this time, it is desirable to suppress temperature fluctuations over the entire length of the rough rolled material at the start of finish rolling to 140° C. or less by heating the rear end of the rough rolled material to a higher temperature than the leading end. This improves the uniformity of product properties in the coil after the winding process.

A known method may be used to heat the rough rolled material. For example, a solenoid type induction heating device is provided between the roughing mill and the finishing mill, and the amount of heating is controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of this induction heating device. may

The hot-rolled and coiled steel sheet is preferably annealed after being subjected to treatment such as degreasing according to a known method, if necessary. Annealing performed on a hot-rolled steel sheet is called hot-rolled sheet annealing, and a steel sheet after hot-rolled sheet annealing is called hot-rolled annealed steel sheet. Descaling by pickling or the like may be performed before hot-rolled sheet annealing.

The heating temperature in the hot-rolled sheet annealing step is preferably higher than 650°C. This is to increase the Mn content and Cr content in the iron carbide in the metallographic structure of the hot-rolled and annealed steel sheet. The heating temperature in the hot-rolled sheet annealing step is more preferably over 680°C, and even more preferably over 700°C. On the other hand, if the heating temperature in the hot-rolled steel sheet annealing step becomes too high, the metal structure of the hot-rolled and annealed steel sheet becomes coarse, and the tensile strength after hot stamping decreases. Therefore, the upper limit of the heating temperature in the hot-rolled sheet annealing step is more preferably less than 750°C, and even more preferably less than 720°C.

In order to sufficiently obtain the effect of hot-rolled sheet annealing, it is preferable to maintain the heating temperature for 30 minutes or longer. On the other hand, if the holding time is too long, the metal structure of the hot-rolled and annealed steel sheet becomes coarse, and the tensile strength after hot stamping decreases. Therefore, the holding time at the heating temperature in the hot-rolled sheet annealing step is preferably less than 10 hours, more preferably less than 5 hours, and even more preferably less than 2 hours.

After the hot-rolled sheet annealing step described above, the hot-rolled annealed steel sheet is preferably cold-rolled to obtain a cold-rolled steel sheet having a thickness of 2.8 mm or less. In order to reduce the weight of the hot-stamped product, the thickness of the cold-rolled steel sheet is preferably 2.3 mm or less, more preferably 2.0 mm or less, and particularly preferably 1.8 mm or less. It is much more preferable that it is 1.6 mm or less. Further, from the viewpoint of steel sheet manufacturability, the thickness of the cold-rolled steel sheet is preferably 0.6 mm or more.

Cold rolling may be carried out according to a conventional method, and descaling by pickling or the like may be carried out before cold rolling. In cold rolling, in order to refine the metal structure after hot stamping and increase the tensile strength, the cold reduction (cumulative reduction in cold rolling) is preferably 30% or more, preferably 40% or more. is more preferable. If the cold reduction is too high, the toughness after hot stamping deteriorates. Therefore, the cold compression rate is preferably 65% or less, more preferably 60% or less. As will be described later, when continuous annealing is performed after cold rolling, the cold reduction rate is preferably 60% or more, more preferably 70% or more, in order to refine the metal structure of the annealed steel sheet. .

An annealed steel sheet may be obtained by subjecting a cold-rolled steel sheet to continuous annealing. The continuous annealing may be performed according to a conventional method, and before the continuous annealing, a treatment such as degreasing may be performed by a known method. In order to refine the metal structure of the annealed steel sheet by recrystallization, the soaking temperature in the continuous annealing is preferably 600° C. or higher, 650° C. or higher, or 700° C. or higher.

On the other hand, if the heating rate during continuous annealing is too slow, the soaking temperature is too high, or the soaking time is too long, grain growth will cause the metallographic structure of the annealed steel sheet to become coarse, and the tensile strength after hot stamping will decrease. descend. Therefore, the average heating rate to the soaking temperature in annealing is preferably 1 ° C./sec or more, the soaking temperature is preferably 800 ° C. or less or 760 ° C. or less, the soaking time is less than 300 seconds, Alternatively, it is preferably less than 120 seconds.

The hot-rolled and annealed steel sheets, the cold-rolled steel sheets and the annealed steel sheets thus obtained may be subjected to temper rolling according to a conventional method.

The steel sheet for hot stamping according to the present embodiment may have a plating layer on the surface layer for the purpose of preventing scale formation during hot stamping and improving the corrosion resistance of the steel sheet after hot stamping. The type of plating is not particularly limited as long as it meets the above purpose, but hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminum-plated steel sheet, hot-dip Zn-Al alloy-plated steel sheet, hot-dip Zn-Al- Mg alloy-plated steel sheet, hot-dip Zn-Al-Mg-Si alloy-plated steel sheet, electro-galvanized steel sheet, and electro-Ni-Zn alloy-plated steel sheet are exemplified.

When a hot-dip plated steel sheet is produced, the hot-rolled annealed steel sheet, the cold-rolled steel sheet, or the annealed steel sheet produced by the above-described method may be used as the material steel sheet and plated according to a conventional method. When a cold-rolled steel sheet is used as the material steel sheet, the soaking temperature in the annealing process of continuous hot-dip plating is set to 600° C. or higher, 650° C. or higher, or 700° C. or higher in order to refine the metal structure of the plated steel sheet by recrystallization. is preferred.

On the other hand, if the soaking temperature is too high, the metal structure of the annealed steel sheet becomes coarse due to grain growth. It is preferable to: After hot-dip plating, the steel sheet may be reheated for alloying treatment.

When producing an electroplated steel sheet, the hot-rolled annealed steel sheet, cold-rolled steel sheet, or annealed steel sheet manufactured by the above-described method is used as the material steel sheet, and if necessary, a known pretreatment for surface cleaning and adjustment is performed. After application, electroplating may be performed according to a conventional method. The plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

(Example 1)
Molten steel was cast using a vacuum melting furnace to produce steels A to N having chemical compositions shown in Table 1. Ac ₁ point and Ac ₃ point in Table 1 were obtained from changes in thermal expansion when cold-rolled steel sheets of steels A to N were heated at 2°C/sec. The Ar ₃ points in Table 1 were obtained from the change in thermal expansion when the cold-rolled steel sheets of steels A to N were heated to 920°C and then cooled at 10°C/sec. Steels A to N were heated to 1200° C. and held for 60 minutes, and then hot rolled under the hot rolling conditions shown in Table 2.

Specifically, steels A to N were subjected to 10 passes of rolling in a temperature range of Ar ₃ or higher to obtain hot-rolled steel sheets having a thickness of 3.6 mm. After hot rolling, the hot-rolled steel sheet is cooled to 540 to 580° C. with a water spray, the cooling end temperature is taken as the coiling temperature, and the hot-rolled steel sheet is charged into an electric heating furnace held at this coiling temperature. It was held for 60 minutes, after which the hot rolled steel sheet was furnace cooled to room temperature at an average cooling rate of 20°C/hour to simulate slow cooling after coiling.

After slow cooling, some of the hot-rolled steel sheets were subjected to hot-rolled sheet annealing. Specifically, the hot-rolled steel sheet is heated to 710°C at an average heating rate of 50°C/hour using an electric heating furnace, held for 1 hour, and then cooled at an average cooling rate of 20°C/hour. A hot-rolled and annealed steel sheet was obtained.

A hot-rolled steel sheet and a hot-rolled annealed steel sheet were pickled to form a base material for cold rolling, and cold-rolled at a cold reduction rate of 61% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. Some of the cold-rolled steel sheets were heated to the soaking temperature for annealing shown in Table 2 at an average heating rate of 10°C/sec using a continuous annealing simulator and soaked for 60 seconds. Subsequently, after cooling to 400° C. and holding for 180 seconds, it was cooled to room temperature to obtain an annealed steel sheet. Regarding the obtained annealed steel sheets, "ACR" is indicated in the "Steel type" column in Table 3, and "-" is indicated in the "Plating type" column. As for the cold-rolled steel sheets, "CR" is indicated in the "Steel type" column in Table 3, and "-" is indicated in the "Plating type" column.

Also, some of the cold-rolled steel sheets were heated to the soaking temperature for annealing shown in Table 2 at an average heating rate of 10° C./sec using a hot-dip plating simulator and soaked for 60 seconds. Subsequently, it was cooled and immersed in a hot-dip galvanizing bath or hot-dip aluminum plating bath to apply hot-dip galvanizing or hot-dip aluminum plating to obtain a hot-dip galvanized steel sheet or hot-dip aluminum-plated steel sheet. Some of the steel sheets were subjected to alloying treatment by heating to 520° C. after hot-dip galvanizing to obtain alloyed hot-dip galvanized steel sheets. The obtained plated steel sheets are described as "ACR" in the "Steel type" column in Table 3, and as "GI", "GA" or "AL" in the "Plating type" column.

From the cold-rolled steel sheet, annealed steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and hot-dip aluminum-coated steel sheet (these steel sheets are collectively referred to as hot stamping steel sheets) obtained in this way, the structure Observation specimens were collected and their structures were observed.

Specifically, in the case of non-plated steel sheets (cold-rolled steel sheets and annealed steel sheets), after polishing the thickness cross-section parallel to the rolling direction, 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface of the steel plate), in the case of plated steel plate, from the boundary between the base steel plate and the plating layer to the plate thickness of the base steel plate From the 1/4 depth position (1/8 depth of the plate thickness of the steel plate as the base material from the above boundary to 3/8 depth of the plate thickness of the steel plate as the base material from the above boundary) by the replica method Precipitates were extracted and TEM was used to identify iron carbides. Quantitative analysis of the three elements of Fe, Mn and Cr was performed on ten iron carbides using EDS. [Mn] _θ and [Cr] _θ are the Mn content (atomic %) and the Cr content (atomic %) in the iron carbide when the total content of Fe, Mn and Cr is 100 atomic %. , [Mn] _θ and [Cr] _θ were averaged.

Also, a JIS 13B tensile test piece was taken from the hot stamping steel sheet along a direction perpendicular to the rolling direction, and a tensile test was performed at a tensile speed of 10 mm/min to determine the tensile strength. Table 3 shows the results of observing the metallographic structure of the steel sheets for hot stamping and the results of investigating the mechanical properties of the steel sheets for hot stamping.

A hot stamping base plate having a width of 240 mm and a length of 170 mm was taken from the above steel plate for hot stamping, and a hat member having the shape shown in FIG. 1 was manufactured by hot stamping. In the hot stamping process, a gas heating furnace is used to heat the base plate at the heating temperature shown in Table 4 for 4 minutes, then it is removed from the heating furnace and allowed to cool, and the starting temperature shown in Table 4 is provided with a cooling device. It was sandwiched between molds to form a hat, and then cooled in the mold to the cooling stop temperature shown in Table 4. Also, some of the hat members were reheated under the conditions shown in Table 4 using an electric heating furnace. The RT for the hot stamping conditions in Table 4 indicates room temperature, and "-" indicates that no reheating step was performed.

A portion of the hat member (hot stamped product) was heat-treated at 170° C. for 20 minutes using an electric heating furnace.

A test piece for SEM observation was taken from the vertical wall portion of the hat member before the heat treatment, and after polishing the plate thickness section parallel to the rolling direction of the steel plate of this test piece, the plate thickness section was subjected to nital corrosion and repeller corrosion. In the case of non-plated steel sheet, the depth position of 1/4 of the thickness of the steel sheet from the surface of the steel sheet (area from 1/8 of the thickness of the steel sheet to 3/8 of the thickness from the surface of the steel sheet) , In the case of a plated steel sheet, the position at a depth of 1/4 of the thickness of the base steel sheet from the boundary between the base steel sheet and the plating layer (1/8 depth of the base steel sheet thickness from the above boundary The metallographic structure was observed in a region from the above boundary to a depth of 3/8 of the plate thickness of the base steel plate). Using the method described above, the area ratios of ferrite, pearlite, retained austenite, tempered martensite, martensite and bainite were measured by image processing. Table 4 shows the results. The remainder of the structure shown in Table 4 was pearlite, retained austenite and/or precipitates. Moreover, in the test numbers satisfying the provisions of the present invention in the table, the proportion of polygonal ferrite in ferrite was 5.0% or more in the metallographic structure of the hot-stamped product.

Also, a JIS 13B tensile test piece was taken along the longitudinal direction of the hat member from the vertical wall portion of the hat member before and after the heat treatment, and a tensile test was performed at a tensile speed of 10 mm/min to determine the tensile strength. The difference (ΔTS) between the tensile strength of the hat member that has not been heat-treated and the tensile strength of the hat member that has been heat-treated is determined. It was judged.

When the tensile strength before the heat treatment was 440 MPa or more and less than 700 MPa and the ΔTS was 100 MPa or less, it was judged as acceptable because it satisfied the regulations of the present invention. On the other hand, if the tensile strength before the heat treatment was less than 440 MPa or 700 MPa or more, or if the ΔTS was more than 100 MPa, it was judged to be unacceptable as not satisfying the provisions of the present invention.

Table 4 shows the results of observing the metal structure of the hat member and the results of evaluating the mechanical properties of the hat member. In Tables 1 to 4, the underlined values are outside the scope of the present invention or outside the preferred production conditions.

Test numbers 1 to 3, 9 to 11, 14 to 17 and 25 to 33, which satisfy the provisions of the present invention, all have a tensile strength of 440 MPa or more and less than 700 MPa, and good strength characteristics , and ΔTS is 100 MPa or less, indicating good thermal stability.

In addition, in the manufacturing process of steel sheets for hot stamping, test numbers 2, 10, 16, 25 and 27 in which hot-rolled sheet annealing was performed had a ΔTS of the hot stamped product of 30 MPa or less, and the thermal stability was particularly good. Met.

On the other hand, in test numbers 20 to 24 of comparative examples using steel sheets whose chemical composition is outside the scope of the present invention, the tensile strength of the hot stamped product is less than 440 MPa, and the strength characteristics are poor, or , ΔTS was 100 MPa or more, and the thermal stability was poor.

Specifically, in test number 21 using steel E, the Mn content of the steel was too low, so the tempered martensite area ratio was insufficient in the metal structure of the hot stamped product, and the tensile strength of the hot stamped product was low. was low.

In Test No. 20 using steel D, the Mn content of the steel was too high, so the tensile strength of the hot stamped product was 700 MPa or more and ΔTS was large.

In Test No. 22 using steel F, the C content of the steel was too high, so the ferrite area ratio was insufficient in the metal structure of the hot stamped product, the tensile strength of the hot stamped product was 700 MPa or more, and ΔTS was It was big.

Test number 23 using steel G had a large ΔTS because the Si content of the steel was too high.

Test number 24 with steel H had a large ΔTS because the B content in the steel was too low.

Although the chemical composition is within the scope of the present invention, test numbers 4 to 8, 12, 13, 18 and 19 of comparative examples in which the manufacturing conditions of the hot stamped article are outside the scope of the present invention are the tensile strength of the hot stamped article. Either the strength was less than 440 MPa and the strength properties were poor, or the ΔTS was 100 MPa or more and the thermal stability was poor.

Specifically, Test Nos. 4 and 5 with Steel A had either too long a hold time in the reheating step or too high a reheat temperature, resulting in low tensile strength of the hot stamped part.

Test No. 6 using steel A had low tensile strength because the heating temperature in the heating process was too low.
Test Nos. 7 and 8 with Steel A had low tensile strength due to either too low forming start temperature or too high cooling stop temperature in the hot stamping process.

In test numbers 12 and 13 using steel B, the holding time in the reheating process was too short or the reheating temperature was too low, resulting in an insufficient tempered martensite area ratio in the metallographic structure of the hot stamped product. ΔTS was large.

Test No. 18 using steel C had a large ΔTS because the forming start temperature in the hot stamping process was too high. Test No. 19 using steel C had an insufficient tempered martensite area ratio and a large ΔTS because the reheating process was not performed.

(Example 2)
Molten steel was cast using a vacuum melting furnace, and steels A to C having chemical compositions shown in Table 1 were produced in Example 1. Steels A to C were hot-rolled, hot-rolled sheet annealed, cold-rolled, and annealed under the conditions shown in Table 5 in the same manner as in Example 1, and then plated to form hot-dip galvanized steel sheets. , alloyed hot-dip galvanized steel sheets, and hot-dip aluminum-coated steel sheets (hot stamping steel sheets) were manufactured.

The metal structures and mechanical properties of these hot stamping steel sheets were investigated in the same manner as in Example 1. Table 6 shows the results of observing the metallographic structure of the hot stamping steel sheet and the results of investigating the mechanical properties of the hot stamping steel sheet.

From these hot stamping steel sheets, a hot stamping blank having a thickness of 1.4 mm, a width of 240 mm and a length of 170 mm was taken. This base plate was joined to a joining steel plate of the same size by laser welding to produce a joined steel plate having a thickness of 1.4 mm, a width of 240 mm and a length of 340 mm. The steel plate for joining has a chemical composition of 0.21% C-0.13% Si-1.31% Mn-0.012% P-0.0018% S-0.043% sol. A cold-rolled steel sheet of Al-0.0030%N-0.21%Cr-0.0018%B was used.

The bonded steel plates were hot-stamped under the conditions shown in Table 7 in the same manner as in Example 1 to produce a hat member having the shape shown in FIG. After that, a portion of the hat member was subjected to heat treatment at 170° C. for 20 minutes using an electric heating furnace.

Then, in the hat members before and after the heat treatment, the metal structures and mechanical properties of the portions made of steels A to C were investigated in the same manner as in Example 1. Table 7 shows the results of observing the metallographic structure of the hat member (hot stamped product) and the results of evaluating the mechanical properties of the hat member.

All test results of test numbers 34 to 36 show that the hot stamped product has a tensile strength of 440 MPa or more and less than 700 MPa, and a ΔTS of 100 MPa or less, indicating good strength characteristics and thermal stability. . The tensile strength of the joining steel plate portion of the hat member was 1545 MPa, 1540 MPa and 1536 MPa for test numbers 34 to 36, respectively.

According to the present invention, it is possible to obtain a hot-stamped molded product with excellent thermal stability, which has a portion with a tensile strength of 440 MPa or more and less than 700 MPa, which has a small change in strength due to paint baking treatment. .

Claims (7)

A hot stamped article,
All or part of the hot stamped article,
in % by mass,
C: 0.001% or more and less than 0.090%,
Si: less than 0.50%,
Mn: 0.50% or more and less than 1.70%,
P: 0.200% or less,
S: 0.0200% or less,
sol. Al: 0.001 to 2.500%,
N: 0.0200% or less,
B: 0.0002 to 0.0200%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
Zr: 0 to 0.300%,
Cr: 0 to 2.00%,
Mo: 0-2.00%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
Ca: 0 to 0.0100%,
Mg: 0-0.0100%,
REM: 0 to 0.1000%,
Bi: 0 to 0.0500%, and the balance: having a chemical composition of Fe and impurities,
The metal structure, in area %,
Ferrite: more than 50.0%,
Tempered martensite: 5.0% or more and less than 50.0%,
Martensite: 0% or more and less than 10.0%,
Bainite: 0% or more and less than 20.0%,
Tensile strength is 440 MPa or more and less than 700 MPa,
ΔTS, which is the amount of decrease in tensile strength, is 100 MPa or less when heat-treated at 170 ° C. for 20 minutes.
Hot stamped moldings. The chemical composition, in mass %,
Ti: 0.001 to 0.300%,
Nb: 0.001 to 0.300%,
V: 0.001 to 0.300%,
Zr: 0.001 to 0.300%,
Cr: 0.001 to 2.00%,
Mo: 0.001 to 2.00%,
Cu: 0.001 to 2.00%,
Ni: 0.001 to 2.00%,
Ca: 0.0001 to 0.0100%,
Mg: 0.0001-0.0100%,
REM: 0.0001-0.1000%, and Bi: 0.0001-0.0500%
containing one or more selected from the group consisting of
The hot stamped article according to claim 1. having a plating layer on the surface,
The hot-stamped article according to claim 1 or 2. A method for producing a hot stamped article according to claim 1 or 2,
A heating step of heating a steel sheet for hot stamping having the chemical composition according to claim 1 or 2 to a temperature exceeding Ac ₃ point;
A hot stamping step of starting hot stamping at a temperature of (Ar ₃ point - 200 ° C.) or more and less than Ar ₃ point on the steel plate for hot stamping after the heating step, and then cooling to a temperature of less than 90 ° C. When,
A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding it at that temperature for 3 to 120 minutes;
A method for manufacturing a hot stamped article. A method for producing a hot stamped article according to claim 1 or 2,
A joining step of joining a steel plate for hot stamping having the chemical composition according to claim 1 or 2 to a joining steel plate to form a joining steel plate;
A heating step of heating the joined steel plates after the joining step to a temperature exceeding the Ac ₃ point of the hot stamping steel plate;
Hot stamping is started on the bonded steel sheets after the heating process at a temperature of (Ar ₃ point - 200 ° C.) or higher and less than Ar ₃ point of the hot stamping steel sheet, and then cooled to a temperature of less than 90 ° C. a hot stamping process to
A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding it at that temperature for 3 to 120 minutes;
A method for manufacturing a hot stamped article. A method of manufacturing a hot stamped article according to claim 3, comprising:
A heating step of heating a steel sheet for hot stamping having the chemical composition according to claim 1 or 2 and having a plating layer on the surface to a temperature exceeding Ac ₃ point;
A hot stamping step of starting hot stamping at a temperature of (Ar ₃ point - 200 ° C.) or more and less than Ar ₃ point on the steel plate for hot stamping after the heating step, and then cooling to a temperature of less than 90 ° C. When,
A method for producing a hot stamped molded product, comprising: a reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140° C. and holding at that temperature for 3 to 120 minutes. A method of manufacturing a hot stamped article according to claim 3, comprising:
A joining step of joining a steel plate for hot stamping having the chemical composition according to claim 1 or 2 and having a plating layer on the surface thereof with a joining steel plate to form a joined steel plate;
A heating step of heating the joined steel plates after the joining step to a temperature exceeding the Ac ₃ point of the hot stamping steel plate;
Hot stamping is started on the bonded steel sheets after the heating process at a temperature of (Ar ₃ point - 200 ° C.) or higher and less than Ar ₃ point of the hot stamping steel sheet, and then cooled to a temperature of less than 90 ° C. a hot stamping process to
A reheating step of heating the molded product after the hot stamping step to a temperature of 100 to 140 ° C. and holding it at that temperature for 3 to 120 minutes;
A method for manufacturing a hot stamped article.

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